Breast cancer patients with high levels of vitamin D are more likely to survive the disease than patients with low vitamin D levels, reports a new review.

Researchers analyzed five studies that included 4,443 breast cancer patients who were followed for an average of nine years. Those with high levels of vitamin D (an average of 30 ng/ml) in their blood were twice as likely to survive as those with low levels (an average of 17 ng/ml, which investigators note, is the average blood level of vitamin D for breast cancer patients in the US).

“Vitamin D metabolites increase communication between cells by switching on a protein that blocks aggressive cell division,” says study author Cedric F. Garland, DrPH. “As long as vitamin D receptors are present, tumor growth is prevented and kept from expanding its blood supply. Vitamin D receptors are not lost until a tumor is very advanced. This is the reason for better survival in patients whose vitamin D blood levels are high.”

“There is no compelling reason to wait for further studies to incorporate vitamin D supplements into standard care regimens since a safe dose of vitamin D to achieve high serum levels above 20 nanograms per milliliter has already been established,” says Garland.